1. Technical Field
The present invention relates to an arc-welding method and an arc-welding apparatus for welding by alternately making a short-circuit state and an arc state while repeating a forward feed and a reverse feed for feeding a welding wire as a consumable electrode.
2. Background Art
Generally, widely used methods for welding galvanized steel sheets are short-circuit transfer welding (e.g. CO2 welding and MAG welding), and pulse MAG welding. FIG. 8, FIG. 11A, and FIG. 11B are diagrams used to explain conventional arc-welding methods for welding galvanized steel sheets, for example. FIG. 8 shows a section of a bead when galvanized steel sheets are welded by a consumable electrode type arc-welding method, which is a typical welding method.
The boiling point of zinc in galvanization 27 on the surfaces of objects to be welded 25 is 907° C., which is lower than the melting point of iron, 1536° C. When arc welding is performed on a galvanized steel sheet, zinc vaporizes, and this zinc vapor attempts to pass through a molten pool and diffuse to the outside. However, when the solidification speed of the molten metal of objects to be welded 25 is high, the zinc vapor cannot be released to the outside of the molten pool sufficiently and remains, as porosity 28, inside or on the surface of welding bead 26. Porosity 28 remaining in welding bead 26 forms a blow hole. Porosity 28 remaining opened to the surface of welding bead 26 forms a pit. Both of a blow hole and a pit affect the strength of welding. For this reason, in the automobile industry, where a large number of galvanized steel sheets are used, blow holes and pits need to be suppressed, and especially the amount of pits is predetermined and controlled in many cases.
As a conventional method for arc-welding galvanized steel sheets, FIG. 11A and FIG. 11B show waveforms of a welding current and a wire-feeding speed when pulse welding is performed with Ar, or a mixture gas containing Ar and carbon dioxide gas in an amount of 25% or less added thereto. FIG. 11A shows a waveform of the welding current; FIG. 11B shows a waveform of the wire-feeding speed.
As shown in FIG. 11A, the waveform of the welding current includes first period TL and second period TH. The sum of first period TL and second period TH is waviness period TW. Waviness period TW is a period in which a number of cycles are repeated in one second. In a known arc-welding method (see Japanese Patent Unexamined Publication No. 1106-285643, for example), the current waveform pattern or the wire-feeding speed is changed at frequencies of 10 to 50 Hz in waviness period TW.
First period TL is a period of a current waveform in which first average arc force FL acts on the molten pool. Second period TH is a period of a current waveform in which second average arc force FH, which is larger than first average arc force FL, acts on the molten pool.
The arc force acts as the force that pushes down the molten pool. Thus, changing first average arc force FL and second average arc force FH makes the molten pool in a waving state. In this waving state, any porosity 28 generated from the galvanized layer in the molten pool reaches the surface of the molten pool because of the flow of the molten pool and the buoyancy of porosity 28, and is released to the outside of the molten pool.